This invention relates to mechanical removal of a plastic layer bonded to a metal tube, and more particularly to such removal of the plastic layer without damage to the corrosion resistant layer bonded to the metal tube and/or to the bare metal tube outer surface.
In the automotive industry, it is typical to create what are called xe2x80x9cISOxe2x80x9d or xe2x80x9cSAExe2x80x9d flared endforms on high pressure fluid conduits, particularly brake tubes. Automotive manufacturers mandate that the front and back flare faces be free from substantial polymeric layers. The manufacturers are concerned over a potential loss of assembly torque over the long term of a vehicle""s life which could occur if the relatively soft polymeric material, eg. nylon, should extrude out of the sealing area and fitting compression area after assembly.
The polymeric material is present on the outer surfaces of the fluid conduits in order to greatly enhance the corrosion resistance of the metal tubing comprising the conduits. Thus, manufacturers of these conduits, especially when end use will be under a vehicle body, do not want to remove any more of this corrosion resistance-enhancing polymeric material than is necessary, eg. for example, not substantially past a flare into the straight section of the tube. Further, the metal tubing generally has a corrosion resistant layer bonded to the metal tube outer surface. As such, it is highly desirable that any removal process not damage the corrosion resistant layer beneath the polymer, nor the outer surface of a bare metal tube (if no corrosion resistant layer has been bonded thereto).
Several methods have been tried, with varying levels of success. However, each method has serious drawbacks, substantially preventing the use thereof. A rotary lathe cut method uses a chuck holder with lathe style square tool bits. It is mounted on a standard rotary head deburring unit. The method is simple and reliable; however, results revealed that the nylon does not easily machine off. Strings/burrs are left on, particularly at the transition line. Further, infinite adjustments and measurements would have to be made due to the tube O.D. variation and eccentricity, to attempt to prevent cutting through the tube substrate.
A method using rotary brushes employs the use of a grinding wheel head driving a brush. The tube was held and rotated by hand. The area where the coating is to be removed is forced into the brush, and the transition line is determined by locating a protective sleeve over the tube at the desired location. The sleeve used is about 2xe2x80x3 long and held in place with a set screw. Unfortunately, this method requires a complex adjustment mechanism to compensate for brush wear. Further, it is very difficult to determine if only the nylon would be removed. Other brushes have been tried with no real success.
A square head die method uses a blunt nose die that has a fixed diameter hole that goes over the steel tube and pushes the nylon to a desired distance. The xe2x80x9cpushed backxe2x80x9d nylon material is then cut off and removed via a rotary lathe cut system. This method has some drawbacks, including but not limited to that it would be extremely difficult, if not impossible to predict the amount (thickness) of nylon removed or conversely, left on, and the rotary lathe would dig into any corrosion resistant coating.
A water jet knife method involves the use of a high pressure water jet system to cut and blow off a nylon coating without affecting a ZnAl substrate. The method involves rotating the tube at high rpm while a stationary high pressure water jet removes the coating. The travel speed of the jet was 12xe2x80x3/minute (=0.2xe2x80x3/sec.). The entire system uses 50 Hp of power (=37.3 KW). This method does appear to remove the nylon coating without affecting the corrosion resistant coating, it is forgiving to the O.D. and ovality variances and has a very fast cycle time. However, it is difficult and costly to have high rpm tube rotation; the system uses ultra clean water as the removal medium (which is expensive), and it is not economical to recycle the water. The water will be a problem to the exposed ends of the tube. A further drawback is that there are high maintenance costs for the system.
Thus, it would be desirable to provide a method for the fast, efficient, precise and cost effective removal of one or multiple polymeric layers from a metal substrate. Further, it would be desirable to provide such a method which will not damage the substrate or any corrosion resistant coating thereon. It would still further be desirable to provide such a method which is forgiving to outer diameter and ovality variances.
The present invention addresses and solves the above-mentioned problems by providing a method for preparing a multi-layer metal tube, comprising the steps of: breaking the adhesion between at least a portion of a polymeric layer and an outer surface circumference of a metal tube having an optional corrosion resistant layer applied thereto, thereby causing the polymeric layer portion to retract and loosely adhere to a non-retracted polymeric layer adjacent the polymeric layer portion; and mechanically removing the retracted portion of the polymeric layer while leaving the corrosion resistant layer intact.
Other applications of the present invention will become apparent to those skilled in the art when the following description of the best mode contemplated for practicing the invention is read in conjunction with the accompanying drawings.